This invention relates to displays for electronic devices and, more particularly, to apparatus for backlighting liquid crystal display screens, and the like. The invention also relates to stand-alone backlighting apparatus, and backlighting apparatus incorporated into display panels, digitizing tablets, and sensible display panels.
Electronic devices quite often include a display screen for providing the user with information about the device and its operating functions. Sometimes, it is only a few digits as in the case of a calculator. Often, the display is a large alpha-numeric and graphics display such as employed with laptop, notebook, and other personal computing devices. When power is no object, the display can be of a kind that is self-illuminating. When power is a consideration, however, the display is very often of the liquid crystal type which consumes very low power. A liquid crystal display (LCD) uses polarization techniques to block or pass light through the pixel positions of the display. Thus, unless there is a backlighting system associated with the LCD, incident light must be reflected back through the LCD in order for the displayed information to be visible. Also, LCDs have a viewing axis or angle at which they provide the best viewing. Early LCDs employed for computer screens in laptop computers were difficult to view except under ideal conditions. If the angle of viewing was even slightly off the viewing axis or the incident light was at a wrong angle, the display was practically unreadable. When the incident light fell below a certain level, the display was unreadable in actuality. The advent of newer LCD materials such as the so-called "twist" screen increased the readability of LCDs; but, it was only a matter of degree. Of course, a reflective LCD is useless in the dark.
The answer to the use of LCDs for computer displays lies in the use of backlighting. In this way, the need for incident light to reflect back through the LCD is eliminated as light from the backlighting panel behind the LCD passes through the LCD, thus making the displayed information visible. A typical backlighting arrangement as employed in laptop and notebook type computers is depicted in FIG. 1. The backlighting panel 10 comprises fluorescent tubes 12 located along opposite edges of a transparent plastic panel 14. The plastic panel 14, as is characteristic of such materials, acts as a light pipe to conduct the light 16 from the fluorescent tubes 12. By putting grooves 18, or the like in the plastic panel 14 at spaced intervals, the grooves 18 are illuminated by the "piped" light 16 thereby causing the panel 14 to glow and emanate light 16' through the LCD 20.
Such fluorescent backlighting panels 10 work substantially for their intended purpose. They do, however, have certain drawbacks. In a cold environment, the tubes 12 hesitate to fluoresce to their full potential and the LCD 20 may appear dark or with dark strobing bands caused by visible standing waves in the tubes 12. The panels 10 are relatively expensive so that if the backlighting panel 10 has to be replaced in whole or in part, it is a costly undertaking requiring that the computer be taken to a repair facility. There are two other factors of a fluorescent backlighting panel as shown in FIG. 2 that contribute to its lack of applicability for more contemporary computers--power consumption and electrostatic emissions.
As depicted in FIG. 2, many devices presently under development replace the prior alpha-numeric keyboard employed for inputting information with a sensible display 22. The sensible display 22 combines the functions of an LCD 20 with a digitizer tablet 24 so that the display 22 functions much like the so-called "light pen" displays of early computers. Thus, to use the display 22, a user employs a pen cursor 26 on the surface 28 of the display 22. The position of the pen cursor 26 can be determined by any of a number of techniques implemented by the digitizer tablet 24 including electro-magnetic sensing, and electrostatic sensing. The sensing according to these techniques typically requires a grid of some sort to be associated with the digitizer tablet 24 of the display 22. If a backlighting panel 10 as described above employing fluorescent tubes 12 is positioned behind the LCD 20 and grids of the digitizer tablet 24, it can cause problems such as those mentioned above related to electrostatic discharges 30 from the fluorescent tubes 12. If the sensing system is an electrostatic type, the spurious electrostatic energy can interfere with the accurate sensing of the cursor 26. Surprisingly, if the sensing is electro-magnetic, the same problem exists. This is particularly true if it is a powered, cordless cursor. In order to save battery power consumption in such a cordless cursor (which is only a small lithium hearing-aid type battery), the signal output level is very low. Thus, there can be interference from the electrostatic discharges 30 from the fluorescent tubes 12. A backlighting approach which did not emit interfering forces of any kind would, therefore, be of definite benefit in product design.
The new pen-driven and personal assistant type of computers are also very power limited. Whereas laptop and notebook computers employ rechargeable and replaceable battery power packs, a design criteria of the personal assistant devices in particular is the ability to run for weeks on end on two or three AAA or AA type alkaline batteries. Because of the power consumption of prior art backlighting schemes, the first personal assistants will most likely employ reflective rather than backlit displays in order to be able to meet this design criteria.
The personal assistant computing devices have other design criteria which also effect the type of display and digitizing (i.e. position sensing) apparatus which is used. Weight is a factor. Even more of a factor is size--both as to length and width, and to thickness. A typical prior art backlit LCD display panel as employed with a prior art laptop or notebook computer can be an inch or more in thickness. This is far to thick for a personal assistant device in which the whole device is intended to be less than an inch in thickness.
Wherefore, it is an object of this invention to provide a backlighting scheme for liquid crystal displays, and the like, which is very low in its power consumption.
It is another object of this invention to provide a backlighting scheme for liquid crystal displays, and the like, which does not emit electrostatic and other interfering energy.
It is still another object of this invention to provide a backlighting scheme for liquid crystal displays associated with computers, and the like, which is easily and inexpensively repaired in the event of failure of the light-emitting source.
It is yet another object of this invention to provide a backlighting scheme for liquid crystal displays, and the like, which can be incorporated into the liquid crystal display structure.
It is a further object of this invention to provide a backlighting scheme for liquid crystal displays, and the like, employed with pen-driven computing devices which can be incorporated into a digitizing tablet associated with the liquid crystal display.
It is a still further object of this invention to provide a sensible backlit display for use with pen-driven computing devices in which liquid crystal display, a backlighting panel, and a digitizing grid are incorporated into one device.
Other objects and benefits of the invention will be recognized and appreciated by those skilled in the art from the detailed description contained hereinafter when taken in conjunction with the drawing figures which accompany it.